This invention relates to components used in RF load and source pull testing of medium and high power RF transistors and amplifiers such as remotely controlled electro-mechanical impedance tuners.
Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment (FIG. 1). The microwave tuners (2), (4) are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor, (3)) is tested, see ref. 1; this document refers hence to “impedance tuners”, see ref. 2, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits.
Impedance tuners for frequencies in the MHz frequency range consist, in general, of a number of tuning blocks each comprising a variable capacitor and a length of transmission line, see ref. 4, (FIG. 2). This cascade of tunable networks allows creating adjustable reflection factors (impedances) over a certain frequency range (FIG. 3), starting as low as 10 MHz, or even lower, and up to 200 MHz, all depending on the capacitance values, the self-resonance of the capacitors (FIGS. 4 to 7) and the choice of cable lengths between capacitors (FIG. 2). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ)=(Z−Zo)/(Z+Zo) {1}, wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm (see ref. 3). The equivalent is the Voltage Standing Wave Ratio: VSWR=(1+|GAMMA|)/(1−|GAMMA|) {2}.
Prior art MHz range variable capacitor structures are shown in FIGS. 4 to 7. They invariably comprise either parallel plate or coaxial structures. This invention discloses a new variable capacitor structure, based on a spiral layout, which is easier to manufacture and which has a simple capacity of adjusting the maximum capacitance limitation and the parasitic self-resonant frequency.